Chris McCarthy at 202- 478-8862
Finding planets around other stars is getting to be old hat for a handful of astronomers who make it their business. But the newest discovery is among the minority found in a relatively circular orbit, adding to the likelihood that Earth-like planets could exist.
The planet, announced this morning, orbits a star called Tau Gruis about 100 light-years away. The newly detected world is roughly 1.2 times the mass of Jupiter and goes around its star every 3.5 years. It is about the same distance from the star as our asteroid belt is from the Sun.
Only a few planets have been found this far from stars, owing to improved detection methods and the fact that astronomers must typically collected data during a complete orbital cycle to firmly determine the presence of a planet.
The first extrasolar planet was discovered in the mid-1990s. In those early years, many of the planets found were very close to their host stars and in elongated, so-called eccentric orbits. These were the easiest planets to discover. It wasn't clear whether their odd orbits would turn out to be the norm or not, said Chris McCarthy, a researcher at the Carnegie Institution of Washington who participated in the latest find.
"Now it's seeming like there is a sizeable and well-populated class of giant planets that have circular orbits," McCarthy said Monday in a telephone interview with SPACE.com.
The percentage of planets in these more "normal" circular orbits will likely grow as methods are refined and more planets are found farther from their stars, McCarthy said.
Circular orbits for giant planets are important because they open the door to the existence of terrestrial planets like Earth. A gas giant in an elongated orbit -- coming close to the star then zooming far away -- would cross the path of any potential Earth-like planet in an Earth-like orbit, thus making cohabitation of two objects with such dissimilar orbits virtually impossible.
Of the roughly 100 known extrasolar planets, 43 have orbits that are more circular than Pluto, which has the most eccentric orbit of our solar system, McCarthy said. Of these, 13 are incredibly close to their host stars -- hot Jupiters, they're called -- and do not figure prominently in characterizing the typical solar system.
In a statement issued by the Carnegie Institution, the planet around Tau Gruis is billed as the 100th extrasolar planet found. But McCarthy and other experts agree that the real No. 100 is a moving target.
The Tau Gruis planet and all others have been detected by the so-called wobble method, which detects slight movements in a star induced by an orbiting world. A handful of planets, after their initial detection, are later removed from unofficial lists because their signal turns out to have involved something else.
In fact, earlier this month one planet came under scrutiny when an astronomer said he determined the evidence involved not a planet but instead a cool area on the surface of a star, like a sunspot. The planet's discoverers aren't ready to concede, however.
Meanwhile, roughly 100 planets have been detected by the indirect wobble method and most will stand further scrutiny, astronomers agree.
The discovery was made using the Anglo Australian Telescope in New South Wales, Australia under the direction of Hugh Jones of Liverpool John Moores University in the United Kingdom. The work was done in cooperation with the world's most prolific team of planet hunters, led by Geoffrey Marcy of the University of California, Berkeley and Paul Butler of the Carnegie Institution.
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